Network ServicesCCNA 200-301

NetFlow Data Not Reaching Collector

Presenting Symptom

NetFlow data is not being received by the collector, and no flow records appear in the collector's interface.

Network Context

A small branch office with a Cisco ISR 4321 router running IOS XE 16.9. The router is configured to export NetFlow version 9 to a collector at 10.1.1.100 on UDP port 2055. The router has a single WAN interface (GigabitEthernet0/0/0) and a LAN interface (GigabitEthernet0/0/1). The collector is reachable via the WAN.

Diagnostic Steps

Verify NetFlow Export Configuration

show running-config | include flow

flow exporter EXPORTER
 destination 10.1.1.100
 transport udp 2055
flow monitor MONITOR
 exporter EXPORTER
 record netflow ipv4 original-input
interface GigabitEthernet0/0/1
 ip flow monitor MONITOR input

Check that the flow exporter, flow monitor, and interface application are configured. Missing any of these indicates incomplete configuration.

Check NetFlow Statistics

show flow monitor MONITOR statistics

Cache type:                               normal
Cache size:                                  1000
Current entries:                               15
High water mark:                               20
Flows added:                                 500
Flows aged:                                  485
Active flows timeout:                       1800 sec
Inactive flows timeout:                       15 sec
Exporting flows to exporter EXPORTER:         0

If 'Exporting flows to exporter' shows 0, the exporter is not sending data. Check exporter configuration and connectivity.

Verify Exporter Status

show flow exporter EXPORTER statistics

Exporter: EXPORTER
  Destination: 10.1.1.100
  Transport: UDP 2055
  Packets exported: 0
  Packets dropped: 0
  Last failure: No failure

If packets exported is 0, the exporter is not sending. Check destination reachability and UDP port.

Test Connectivity to Collector

ping 10.1.1.100

!!!!!

If ping fails, there is a network connectivity issue. If ping succeeds, the problem is likely a firewall blocking UDP 2055 or a misconfigured collector.

Check for Firewall or ACL Blocking

show access-lists

Extended IP access list BLOCK_NETFLOW
    10 deny udp any any eq 2055
    20 permit ip any any

If an ACL is blocking UDP port 2055, NetFlow packets will be dropped. Look for deny statements matching the destination port.

Root Cause

An inbound ACL on the WAN interface (GigabitEthernet0/0/0) is blocking UDP port 2055, preventing NetFlow export packets from reaching the collector.

Resolution

Modify the ACL to permit UDP port 2055 to the collector. For example: conf t ip access-list extended BLOCK_NETFLOW no 10 permit udp any host 10.1.1.100 eq 2055 10 deny udp any any eq 2055 end This removes the deny statement and adds a permit for the collector before the deny.

Verification

show flow exporter EXPORTER statistics Expected output: Packets exported: 10 (increasing) Also check collector interface to see flow records appearing.

Prevention

1. Use a consistent UDP port for NetFlow and ensure it is permitted in all ACLs. 2. Implement logging on ACLs to detect blocked traffic. 3. Test NetFlow export after any ACL changes.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why NetFlow data is not being exported. The exam tests knowledge of NetFlow configuration components (exporter, monitor, record) and common issues like ACL blocking. Key fact: NetFlow uses UDP, and the destination port must be allowed.

Exam Tips

Remember that NetFlow export uses UDP; the collector must be reachable and the port open.

The 'show flow monitor statistics' command shows if flows are being exported; zero exports indicate a problem.

Always check ACLs on the egress interface if export is not working.

Commands Used in This Scenario

show access-lists

Displays all configured access control lists (ACLs) on the device, including their entries and match counters, used to verify ACL configuration and traffic filtering.

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